Carbon Monoxide. Toxic Gas and Fuel for Anaerobes and Aerobes: Carbon Monoxide Dehydrogenases - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 41

acceptor, but some microorganisms couple CO oxidation to reduction of nitrate or
dinitrogen [ 9 , 25 ].
1.2.2.2 Fates of Carbon Monoxide under Anaerobic Conditions
Phylogenetically diverse anaerobic bacteria and archaea, including sulfate reducers,
hydrogenogens, acetogens, and methanogens employ the Wood-Ljungdahl path-
way (also called reductive acetyl-CoA pathway) to generate biomass and energy
from CO (Figure 1 )[ 18 , 26 , 27 ]. This pathway allows microbes to thrive in anoxic
niches in the presence of CO or CO 2 and H 2 . The Wood-Ljungdahl pathway
consists of two branches, namely a methyl and a carbonyl branch.
Figure 1 The Wood-Ljungdahl pathway. ACS, acetyl-CoA synthase; CODH, Ni,Fe-containing
carbon monoxide dehydrogenase; CoFeSP, corrinoid iron-sulfur protein; MeTr, methyl-
tetrahydrofolate:corrinoid iron-sulfur protein methyltransferase. The figure is modified from
Ragsdale and Pierce [ 26 ].
The methyl branch is present in all organisms as folate-dependent one-carbon
pathway. Here, CO 2 is reduced to formate, which is subsequently bound to
tetrahydrofolate (H 4 F), yielding 10-formyl-H 4 F. After dehydration, the methenyl
group is reduced stepwise, generating 5-methyl-H 4 F[ 26 ]. The methyl-H 4 F:
corrinoid iron-sulfur protein methyltransferase (MeTr) passes the methyl group of
methyl-H 4 F to the corrinoid iron-sulfur protein (CoFeSP), which connects the
methyl and the carbonyl branch (Figure 1 ).
In the carbonyl branch the CODH component of CODH/ACS catalyzes the
reduction of CO 2 to CO, which is condensed to acetyl-CoA by acetyl-CoA synthase
(ACS) with CoASH and the methyl group donated by CoFeSP. Acetyl-CoA
can be used by the microbes as building block for cellular carbon compounds,
to generate acetate (by acetogens) or as a source of energy (by acetoclastic
methanogens) (Figure 1 )[ 26 ].